1. Field of the Invention
This invention relates generally to a flow meter, and more particularly to a variable area obstruction in a conduit for use with a pressure transducer to measure the flow rate of a fluid in that conduit.
2. Prior Art
A number of different types of flow meters are presently available for measuring the flow rate of a fluid in a conduit. These flow meters can be classified broadly into two groups; namely, the group which presents little or no interference to the flow stream and the group which has a significant influence on the flow stream. The first group of devices which present relatively little or no interference to the flow stream are relatively elaborate and expensive systems. The present invention is concerned with the second group of flow meters which are less elaborate and considerably less expensive, but present some significant amount of interference to the flow stream.
This second group of flow meters includes the type which is position and/or displacement oriented and the type having no moving parts. The position and/or displacement type of flow meter includes some movable part which changes its position, displacement or velocity in response to the flow of fluid with respect thereto. Examples of such compliant type of flow meters are the turbine flow meters, the variable capacitance flow meters, and the oscillating vane flow meters. These compliant type of flow meters all have the usual problems associated with moving parts. Furthermore, because of the relatively large pressure drop produced by these flow meters, they are normally not employed in a closed flow system, but rather in an open flow.
The second group of flow meters having no moving parts employ some type of obstruction in the flow stream and a differential pressure transducer for measuring the difference in fluid pressure across the obstruction. The obstruction in this type of flow meter has been formed in the past of either a plate having a fixed area orifice therein, a venturi, or a flow nozzle. The pressure differential across the obstruction is a measure of the flow rate of the fluid passing through the conduit. Under constant pressure and enthalpy conditions, this pressure differential is proportional to the square of the flow rate of the fluid in the conduit. This square law relationship is highly undersirable, since the error at the lower flow rates is a greater percentage of the flow rate than the error at the higher flow rates. It is generally desirable, when measuring flow rates, to have an error which is equal at both the lower flow rates and the higher flow rates. When flow rate is to be employed for measuring accumulated volume, it is desirable to have an error at the lower flow rates which is less than that at the higher flow rates, such that the error associated with accumulated volume is constant with changes in the flow rate.